Optical glass fibers are generally coated with two superposed radiation-cured coatings, which together form a primary coating. The coating which is in direct contact with the glass is called the inner primary coating and the overlaying coating(s) is called the outer primary coating.
The inner primary coating is usually a relatively soft coating providing environmental protection to the glass fiber and resistance, inter alia, to the well-known phenomenon of microbending. Microbending in the coated fiber can lead to attenuation of the signal transmission capability of the coated fiber and is therefore undesirable. The outer primary coating(s), which is on the exposed surface of the coated fiber, is typically a relatively harder coating designed to provide a desired resistance to physical handling forces, such as those encountered when the fiber is cabled.
For the purpose of multi-channel transmission, optical glass fiber assemblies containing a plurality of coated optical fibers have been used. Examples of optical glass fiber assemblies include ribbon assemblies and cables. A typical ribbon assembly is made by bonding together a plurality of parallel oriented, individually coated optical glass fibers with a matrix material. The matrix material has the function of holding the individual optical glass fibers in alignment and protecting the same during handling and the installation environment. Often, the fibers are arranged in "tape-like" ribbon structures, having a generally flat, strand like structure containing generally from about 2 to 24 fibers. Depending upon the application, a plurality of resulting ribbon assemblies can be combined into a cable which has from several up to about one thousand individually coated optical glass fibers. An example of a ribbon assembly is described in published European patent application No. 194891. In general, a plurality of ribbon assemblies may then be combined together in a cable, as disclosed in U.S. Pat. No. 4,906,067.
The term "ribbon assembly" as used herein includes the tape-like ribbon assembly described above, as well as optical glass fiber bundles. Optical glass fiber bundles can be, for example, a substantially circular array having at least one central fiber surrounded by a plurality of further optical glass fibers. Alternatively, the bundle may have other appropriate cross-sectional shapes such as square, trapezoid, etc.
Coated optical glass fibers for use in optical glass fiber assemblies are usually coated with an outer colored layer, called an ink coating, or alternatively a colorant is added to the outer primary coating to facilitate identification of the individual coated optical glass fibers. Thus, the matrix material which binds the coated optical glass fibers together contacts the outer ink layer if present, or the colored outer primary coating.
Ink coatings usually have a thickness of about 3 to about 10 microns and are formed from a pigment dispersed within a UV curable carrier system. The UV curable carrier system contains a UV curable oligomer or monomer that is liquid before curing to facilitate application of the ink composition to the optical glass fiber, and then a solid after being exposed to UV radiation. In this manner, the UV curable ink composition can be applied to a coated optical glass fiber in the same manner as the inner primary and outer primary coatings are applied.
It is commonly required that, in use, branching fiber connections must be made at a location intermediate to the respective termini of a given length of the ribbon assembly. Accessing the individual fibers in this manner is commonly referred to as "mid-span access" and presents special problems. Normal methods and tools for accessing the end or terminus of the ribbon assembly are generally not well adapted or are inoperable for providing midspan access.
There have been many attempts to provide a ribbon unit in which the matrix material is easily separated from the colored coating present on optical glass fibers at any location on the ribbon unit without removal of the colored coating from the coated optical glass fibers. However, if the separation of the matrix material also removes the colored coating from the fibers, the purpose of individual fiber identification will be negated.
One common method for providing mid-span access is to contact the matrix material with a solvent, such as ethanol or isopropyl alcohol. Such a solvent must have the ability of swelling or softening the matrix material. At the same time, the solvent should be selected so as not to swell the coatings on the individual optical glass fibers. The swelling of the matrix material weakens that matrix material so that it can then be mechanically removed by mild scrubbing or similar mechanical means to remove the matrix material and thereby provide access to the individual, but still coated and color-identifiable, optical glass fibers. An example of this solvent stripping method is described in the AT&T brochure "D-182355 Accuribbon.TM. Single Fiber Access" (Mar. 3, 1991).
Published European application number 0614099A2 discloses an optical fiber ribbon unit in which the bonding between the coloring layer of the individual optical glass fibers and the matrix layer is suppressed by adding 5% by weight or less of a release agent to each of the layers. The purpose of adding the release agent is to prevent the coloring layer from being peeled off when the matrix material is separated from the optical glass fibers. Examples of such release agents include a silicone release agent or a fluorine-base release layer.
Published Japanese Patent Application No. 64-22976 discloses radiation-curable ink compositions containing specific radiation-curable oligomers. The ink composition provides an ink coating having adhesion to the outer primary coating which is separable from the matrix material in a ribbon assembly.
Published Japanese Patent Application No. H1-152405 discloses a radiation-curable ink composition containing an organic polysiloxane compound. The polysiloxane compound provides the ink coating with the ability to separate more easily from the matrix material in a ribbon assembly.
U.S. Pat. No. 4,900,126 (Jackson) discloses an optical glass fiber ribbon unit in which each of the individually coated optical glass fibers has a colored outer layer. Each of the optical glass fibers is further coated with a release agent which has a low affinity for the bonding material or the colorant material. An example of the release agent is teflon. The release agent creates a weak boundary layer at the interface of the colorant material and the matrix material whereby the matrix can be separated from the optical glass fibers without removing the colored layer on the individual optical glass fibers.
U.S. Pat. No. 4,953,945 discloses using a peelable cured coating layer between an outer colored layer of optical glass fibers and the matrix material whereby the matrix material can be stripped from the optical glass fibers without removing the colored layer of the optical glass fibers.
U.S. Pat. No. 5,524,164 discloses a cable assembly comprising a plurality of ribbon assemblies. The common coating material that bind the ribbon assemblies together contains a component having poor compatibility with the main component in the common coating. Examples of such poor compatibility components include hydrocarbons having from 10 to 20 carbon atoms, silicone oils and fluorine oils. The poor compatibility component reduces the friction between the ribbon assemblies to prevent damage to the fibers when the cable is bent. The poor compatibility component provides a discontinuous layer on the common coating, in the form of "seas" and "islands". There is no disclosure or teachings relating to mid-span access to the individual coated optical glass fibers contained within the ribbon assemblies.
U.S. Pat. No. 5,561,730 discloses a cable containing a plurality of ribbon units. The common coating material that bind the ribbon assemblies together contains a release agent. Examples of such release agents includes silicone oils and fluorine oils. The release agent reduces the friction between the ribbon assemblies to prevent damage to the fibers when the cable is bent. There is no disclosure or teachings relating to providing mid-span access to the individual coated optical glass fibers contained within the ribbon assemblies.
U.S. Pat. No. 5,621,838 discloses a coated optical glass fiber unit made of a plurality of coated optical glass fibers which are bound together by a common bundling layer. The ink coating on the optical glass fibers and the common bundling layer are treated to suppress bonding there between so that the bundling layer can be removed from the ink layer. A silicone releasing agent or a fluorine releasing agent are added to the ink coating and the common bundling layer.
Silicone and fluorine based release agents can cause undesirable degradation of the inner primary and outer primary coatings over time. When they are not bound to the coating, they can leech out of the coating. They are often time incompatible with the desired coating composition. Thus, conventional silicone and fluorine release agents can only be used in small quantities, such as less than 5% by weight. If the conventional release agents are used in amounts greater than 5% they can cause the matrix material and ink coating to swell and they can collect between the matrix and the ink coating layer causing unavoidable peeling of the matrix from the optical glass fibers, thereby diminishing the protective function of the matrix material.
Use of acrylated silicone and fluorine based release agents in optical glass fiber coatings is also undesirable. For examples, these types of release agents are often incompatible with the components in the ink coating. Use of these types of release agents can also cause undesirable changes in the properties of the ink coating.
As the demand for coated optical glass fibers has increased, manufacturers must respond by adding more fiber drawing production lines and by attempting to increase the linear line speeds of the existing fiber drawing production lines. In the latter case, one factor which will determine the upper limit for the line speed will be the curing rate characteristics of the radiation-curable ink composition, for a given radiation source and intensity.
If the line speed is increased to the extent that cure rate time requirements of the radiation-curable ink composition are not provided, the radiation-curable ink composition will not have received a sufficient amount of radiation to cause complete cure, or cross-linking, of the radiation-curable ink composition. The production linear line speed is generally inversely related to the amount of radiation striking the optical glass fiber. That is, as the production line speed is increased the amount of radiation exposure to the radiation-curable ink composition during the production process will necessarily decrease for a given radiation source. Incomplete cure of the radiation-curable ink composition is undesirable and must be avoided because then the desired properties of the incompletely cured ink coating may not be achieved and/or the incompletely cured ink coating may retain tackiness (giving problems in subsequent handling) or a malodorous odor may be present, and there may also be an increase in the extractables (undesirable) in the supposedly-cured ink coating and a lack of adhesion to the coated optical glass fiber.
While the ink composition must have a very fast cure speed to ensure complete cure of the ink coating on the high speed drawing tower, the increase in cure speed should not come at the expense of other important properties of the ink coating, such as providing suitable mid-span access when used in ribbon assemblies. Therefore, there is a need for a radiation-curable ink composition that exhibits adaptable adhesion properties to provide an adhesion between the outer primary coating and the ink coating that is greater than the adhesion between the ink coating and the matrix material to provide mid-span access.
In addition, ink compositions should not contain ingredients that can migrate to the surface of the optical glass fiber and cause corrosion. The ink composition should also not contain ingredients which can cause instability in the protective coatings. Ink coatings for optical glass fibers should be color fast for decades, not cause attenuation of the signal transmission, be impervious to cabling gels and chemicals, and allow sufficient light penetration for fiber core alignment.
From the above, it is clear that optical glass fiber technology places many unique demands on radiation-curable ink compositions which more conventional technologies, such as printing inks, do not. There is a need for radiation-curable ink compositions that provide mid-span access and which avoid the problems associated with conventional fluorine and silicone based release agents.
Usually ink compositions must be cured in an inert atmosphere, i.e. in the absence of oxygen. Providing inert atmospheres on optical glass fiber drawing towers is expensive. Thus, a radiation-curable ink composition which exhibits a high cure speed in the presence of oxygen would provide significant advantages over ink-compositions that must be cured in an inert atmosphere.